Add ECOVACS GOAT mower documentation

[monsivar]

Summary

Add an initial ECOVACS GOAT mower documentation section to the DeebotUniverse documentation.

This contribution:

• adds a dedicated GOAT Mowers section to the MkDocs navigation
• documents the five GOAT models with dedicated upstream deebot-client hardware profiles
• documents the current upstream mowing-control baseline
• explains the shared clean terminology used by deebot-client
• adds GOAT mowers to the existing model support page

Scope

This first contribution intentionally documents only functionality that is currently present in the upstream DeebotUniverse/client.py dev branch.

It does not document unmerged development work, experimental features, Home Assistant development branches, or reverse-engineering findings that have not reached upstream.

The documented client.py baseline was verified against:

5453eba9b2225093e8b2a41512830d17054518f6

Validation

• git diff --check passes
• navigation targets and relative Markdown links were checked
• mower model and capability claims were verified against the current upstream hardware profiles
• a full MkDocs build was not run because MkDocs is not installed in the local Python environment